Power supply switching IC, or the like, is exclusively used for controlling individual high-voltage devices, and such a control IC is generally connected as an external circuit to the high-voltage devices. If this control IC and the high-voltage devices can be integrated in the same silicon chip, the number of components is reduced, and the resulting power supply system may be simplified.
To integrate the high-voltage devices and control IC in the same silicon chip, there is a need to prevent the control IC from operating improperly due to noises occurring upon switching. To this end, the high-voltage devices are configured to provide lateral structures, and all of their electrodes are taken out from one of opposite surfaces of a semiconductor substrate, while the other surface of the substrate is grounded so that the high-voltage devices and control IC integrated in the semiconductor substrate are connected to a common ground. In the lateral structure, however, a high voltage is applied between electrodes formed on the semiconductor substrate, in the lateral direction of the substrate, and therefore an electric field produced by the applied voltage may concentrate at local portions between the electrodes if foreign matters, or the like, are deposited on the surface of the semiconductor substrate. Such concentration of the electric field may cause reduction of the breakdown voltage of the device. Thus, the surface condition of the semiconductor substrate is extremely important in this type of semiconductor apparatus.
Conventionally, a silicon nitride film having a refractive index of 2.8 or higher, which serves as a conductive protective film, is used in order to protect the surface of the semiconductor apparatus against contamination, electrically stabilize the apparatus, and thereby prevent concentration of the electric field.
In a conventional process, however, a thin silicon oxide film 11 is undesirably formed on the surface of the silicon nitride film 12, as shown in FIG. 6, and charges are accumulated at the interface between this unnecessary thin silicon oxide film 11 and the silicon nitride film 12. As a result, the silicon nitride film 12 has nonuniform or varying resistance over its area, and the electric field concentrates at local areas of this film, resulting in reduced breakdown voltage of the semiconductor apparatus.
If the semiconductor apparatus is operated in a high-temperature, high-humidity atmosphere after the silicon chip is packaged in a mold resin, oxide films 13 are formed in local portions of the surface of the silicon nitride film 12, as shown in FIG. 7, and cause reduction of the breakdown voltage. Such oxide films 13 are formed because the silicon nitride film 12, which is a conductive protective film, is more hydrophilic and more likely to be oxidized than a conventional insulating protective film. FIG. 8 shows the chemical reaction that occurs in this silicon nitride film 12 (refer to semiconductor world 1984, 12 p 168).